2 gauge (also called II gauge) is a model railwaygauge originally 64 mm (21⁄2 in), then standardised in 1909 at 2 in (50.8 mm), a 20% reduction and a change in definition: from mm to inch. It has since fallen into disuse. The gauge was introduced by Märklin at the Leipzig toy fair in 1891. 2 gauge was equivalent to a 1:22.5 scale.
After the change to 2 in (50.8 mm) in 1909, 64 mm (21⁄2 in) gauge was standardised as 3 gauge (or III gauge).
The European G gauge trains are built to the same 2 gauge scale, but with a slightly different rail gauge of 45 mm (1.772 in) (the same as 1 gauge). This G gauge is the 1:22.5 equivalent of 1,000 mm (3 ft 33⁄8 in) metre gauge gauge in modern railroads. As a result, the scale-gauge combination is sometimes called IIm in European literature.
In the UK, No 2 Gauge was 2 in (50.8 mm) while No 3 gauge was 21⁄2 in (64 mm). From this, it follows that G gauge is sometimes, albeit rarely, referred to as 3m. A 2 in (50.8 mm) gauge of standard gauge locos gives a scale of 1:28.25, not so far off the 1/29 used by some manufactures with 45 mm (1.772 in) gauge track. In the grand tradition of model rail gauges often being narrower than the full scale equivalent (00 scale being the classic example) this is deemed perfectly acceptable.